1. Technical Field
The present invention relates to apparatus and methods for accelerating electrons and extracting energy from electron beams or other particle beams, in particular to techniques for generating x-rays.
2. Background Information
X-rays can be generated by accelerating electrons to high speeds. Conventional methods of x-ray generation use a linear particle accelerator to accelerate electrons to energies of several keV or even MeV. A conventional cathode ray tube emits an electron beam that is accelerated towards a target anode where electromagnetic radiation is generated upon impact. Soft x-rays having energies of 0.12 to 12 keV and wavelengths of 10 to 0.1 nm can be generated in this way. In more recent times linear accelerators such as the Stanford Linear Accelerator Center (SLAC) typically achieve electron energies around 3 GeV by using RF fields to progressively accelerate an electron beam as it passes through a vacuum tube containing segmented electrodes. Such high energy electron beams can be circulated in a storage ring using synchronized electric and magnetic fields to provide a source of synchrotron radiation including x-rays. These extremely bright (i.e. high flux) x-rays can be used to investigate molecular structures, resulting in many bio-medical applications such as protein crystallography.
While synchrotron light sources such as the one at SLAC and the Diamond light source in the UK can provide researchers with very hard and bright x-rays for experimental studies, such facilities are extremely large, costly to run and not readily available to everyone. The Diamond light source is housed in a toroidal building that is 738 m in circumference and covers an area in excess of 43,300 m2. Although the x-rays from a synchrotron source can be a billion times brighter than those, for example, generated by cathode ray tubes for normal medical imaging, a synchrotron source converts only a tiny fraction of the energy of the electrons into radiation. Furthermore the natural synchrotron light is not monochromatic and its application, for example, to phase-contrast imaging may require the use of sophisticated insertion devices and other techniques. Alternative x-ray sources are required that can meet academic and industry demands on a more accessible scale.
One alternative to synchrotron light sources is a linear accelerator(linac)-based coherent light source such as the Linac Coherent Light Source (LCLS) at SLAC. This facility couples a linear particle accelerator with a free electron laser (FEL) to produce intense x-rays. In a free electron laser the electron beam itself is used as the lasing medium. The electron beam from the linac is injected into an undulator or “wiggler”—an array of magnets arranged with alternating poles along the light beam interaction path to slightly wiggle the electron beam transversely and stimulate the emission of coherent electromagnetic radiation in the form of x-rays. FEL radiation is monochromatic and extremely bright—the process of self-amplified spontaneous emission extracting a much greater fraction of the electrons' energy than can synchrotron radiation. In fact FEL x-ray sources can be many orders of magnitude brighter than synchrotron light sources.
Some researchers have demonstrated energy recovery in conjunction with a free electron laser by decelerating the electron beam after it passed through a wiggler. The ALICE accelerator at Daresbury Laboratory in the UK has coupled an energy recovery linac to the undulator of a free electron laser generating light in the mid-IR range. In such a proposal the spent electron beam is returned back to the entrance of the main linac via an additional beam path at a precise time when the RF phase is exactly opposite to the initial accelerating phase such that the beam is decelerated and energy can be recovered back to the electromagnetic field inside the linac RF cavities. This energy recovery technique requires an accurate adjustment of the electron beam path length that is accomplished by moving the arc of the beam path as a whole.
While accelerators such as the LCLS at SLAC and ALICE at Daresbury Laboratory have demonstrated the potential of FELs as light sources, there are several drawbacks. Such facilities are extremely large—the LCLS based on a linear accelerator at SLAC, for example, is over 3 km long in total and includes a 600 m linac, 230 m electron beam transport tunnel, 170 m undulator and over 300 m of tunnels to transport x-rays to experimental halls. The overall billion dollar-scale cost and huge size of such machines means that they can only be constructed at a national level. There remains a need for smaller research bodies to have access to their own x-ray source facilities.
Researchers at MIT have recently proposed an alternative x-ray source that is potentially smaller than the LCLS or other sources based on the principle of a free electron laser. This alternative technique uses inverse-Compton scattering to generate x-rays when an electron beam is collided with photons e.g. from a laser beam. U.S. Pat. No. 7,391,850 describes such a laboratory scale x-ray source. In order to generate x-rays having energies on the keV scale, the electron beam is accelerated to energies of the order of tens of MeV using a linear accelerator comprising superconducting RF cavities. The accelerator module is contained in a cryostat operating at a temperature of 2 K. Although smaller than the other light sources discussed above, the accelerator cryomodule is still over 3 m long and requires a significant power supply for the RF cavities in the superconducting accelerator. Much of the input power is wasted as the electron beam is dumped following its interaction with the laser beam.
Another x-ray source that is based on inverse-Compton scattering is the Compact Light Source from Lyncean Technologies. Instead of using a linear accelerator with superconducting RF cavities, an electron beam is circulated in a laboratory-sized storage ring and interacted with a laser beam to generate x-rays. While this source is able to generate x-rays on a much smaller scale than conventional synchrotron light sources, the power requirements are still quite large as a beam injector must be able to accelerate bunches of electrons up to 25 MeV. The electron bunches are then circulated in the storage ring for about one million turns as the interaction rate is very low. While the circulating electron beam can interact with the laser beam a large number of times, it is difficult to create a large electron current in a small ring so the energy efficiency of the process is limited. Electrons that do not interact are dumped after completing their circulation. This method of x-ray generation may therefore be considered to have a low energy efficiency.
While x-ray sources based on inverse-Compton scattering have shown potential in terms of reduced size and cost, there remains a need for a compact x-ray source that can efficiently generate high-energy and high-flux x-rays for use in a wide range of experiments.